Broadening mechanism in semiconductor (GaAs) lasers: Limitations to single mode power emission

A nonlinear steady-state theory of the emission spectrum of semiconductor (GaAs) lasers above threshold is developed, and limitation to power in a single longitudinal mode is studied. The nonlinear steady-state rate equations describing the power and the gain are solved iteratively. The model is based on the well known idea that the gain always saturates somewhere below the loss, and power sharing among the modes is dependent on the relative gain of the modes with respect to the loss level. The limitation to single mode power is essentially due to the uneven rate of saturation of the gain of the different modes as they approach the loss level asymptotically, with the dominant mode having the fastest saturation. The rate of saturation of the gain of different modes depends on the power emission spectrum and the intraband relaxation rate of the carriers. In this work, the relaxation is accounted for by using the generalized spectral weight function to describe the carriers. The dependence of maximum single mode power on intraband relaxation time is obtained. It ranges from a few milliwatts for relaxation time of the order of 10^-12s to hundreds of milliwatts for relaxation time of 10^-13s. The predictions of the model on gain saturation spectra, and carrier lifetime spectra agree well with experimental observations. The gain is seen to saturate near the lasing energy but continues to increase at a reduced rate at higher energy levels. The carrier radiative lifetime is found to decrease sharply in the vicinity of the lasing mode energies.